O(6)-Alkylguanine is believed to be a major mutagenic and possible carcinogenic lesion in DNA caused by alkylating agents such as N-nitrosamines and N-nitrosamides, and the exzymatic repair of this lesion may therefore be an important factor in the potency and organotropism of these carcinogens. We propose to study this DNA repair process in the rat which has been extensively used as a model system for studies of nitrosamine-induced carcinogenesis and in which in vivo repair of O(6)-methylguanine lesions has been correlated with tissue-specific resistance to tumor induction. Our experimental methods are designed to distinguish between possible repair pathways in a cell-free systeem in order to determine which pathways are operative in rat liver and other tissues, and the repair enzyme(s) will be purified in order to study general properties, substrate specificity and enzyme kinetics. A biochemical basis will be sought for the tissue-specific differences in O(6)-methylguanine repair rates in the rat which are believed to be a factor in the organotropism of simple alkylating agents. The induction of O(6)-methylkguanine repair system in rat tissues will be investigated with respect to the level of induction and the mechanism and specificity of induced repair. These fundamental studies will provide a better understanding of the role of O(6)-alkylguanine and its repair in chemical carcinogenesis.